The purpose of this study is to determine if thymic enlargement in HIV-infected patients is secondary to development of new T cells in the thymus or to attack of cytotoxic lymphocytes on the thymus. The thymus is the organ in which new T cells develop. In children the thymus can respond vigorously to produce new T cells if they are destroyed as by HIV or chemotherapy. It is unknown whether the thymus of HIV-infected adults can respond to reconstitute the immune system after the virus is brought under control by highly active antiretroviral therapy including protease inhibitors. The question being addressed was stimulated in part by an autopsy observation in an HIV-infected patient who had a slightly large thymus at the time of death but did not have any developing T cells. Instead the patbologist only found cytotoxic T cells which appeared to be attacking the thymus. The methods used in this study are biopsy of the patient=s thymus followed by immunohistochemistry and flow cytometry evaluation of the material. PCR is used to assess the level of T cell receptor rearrangement excision circles. Peripheral blood is examined for evidence of recent thymic emigrants (newly formed T cells).